Another thing to keep in mind that your legs are likely going to feel tired as you ride more and more until you're adequately warmed up. It take me a good 5-8 miles to warm up at a decent pace. So, if you're cutting your rides short because your legs feel tired, you may be selling yourself short a bit. As you go forward, try to work through the fatigue. I'm not suggesting that you overdo it, but you'll start to sense when you need to take it easy (or take the day off) and when you just need a long warmup.